This invention relates to a method of constructing a thin film mirror.
The use of thin film mirrors in large types of visual display apparatus is well known. Thus, for example, it is well known to use thin film mirrors of a concave near-spherical form in collimated displays for use in simulators. The simulators may be used for a wide variety of purposes including training, research, leisure and entertainment.
The thin film mirrors are usually constructed by fixing the film across an open face of an otherwise enclosed chamber. The edges of the chamber to which the film is attached lie on the surface of the desired mirror shape. In the case of simulator display apparatus, the mirror chamber is usually part of a sphere which is bounded at its top and bottom by lines of latitude. Left and right hand ends of the mirror chamber may be lines of longitude or more complicated arrangements.
When first applied, the film is lightly tensioned such that it forms part of the frustum of a cone. The chamber is then partially evacuated, and the greater external air pressure then forces the film into the desired mirror shape. The accuracy of the mirror shape is a major factor in the quality of the image provided by the display apparatus. The chamber edges, the means by which the film is attached to the chamber edges, and how the film stretches as the chamber is evacuated, are all factors which affect the final shape of the mirror. Various methods are known for improving the accuracy of the mirror shape in these areas.
It will be appreciated that the film must stretch in going from a cone to a sphere. In vertical section, the film must stretch from a straight line to a circular arc. In horizontal section, the film must stretch from a circular arc to a longer circular arc. However, where the film attaches to the fixed chamber edges, the film cannot stretch parallel to the edge. Thus, the stretch is not uniform across the film. The reduced stretch near the edges of the film results in locally lower tension in the film, and a departure from the ideal shape. In vertical section, the film follows a xe2x80x9cbathtubxe2x80x9d curve, with a good curvature over the central region but sharply changing curvature near the edges.
A known process to improve the mirror edge accuracy is to over-stretch the film initially, for up to a few hours, before relaxing the film to its design position. However, this still leaves a significant band of poor mirror around the edge. This band of poor mirror results in objectionable distortion in the image of the display apparatus. To produce an acceptable mirror, it is therefore necessary to make the mirror larger and then cover up the poor edge, thereby leaving the required area of useable mirror in the center.
It will be appreciated that, as the display apparatus becomes larger, it becomes more costly to manufacture the display apparatus. Also, the greater mass of the display apparatus imposes an increasing load on support parts of the display apparatus. Still further, since the display apparatus is usually mounted on some kind of motion system, the motion system expands, requiring a larger and more expensive building to house the display apparatus, for example a simulator. Film of adequate quality is only available up to a certain width, and this also limits the size of the display apparatus. Such limitations usually result in compromises in the size of the covered edge, so that some distortion is still seen around the edge of the image. Vertical linearity of the image may also be compromised.
It is an aim of the present invention to provide a method of constructing a thin film mirror, which method results in a thin film mirror having an improved edge quality without the need for a substantial increase in the size of the thin film mirror.
Accordingly, the present invention provides a method of constructing a thin film mirror, which method comprises:
(i) providing a suction chamber with edges which lie on a surface of the thin film mirror being constructed;
(ii) providing tensioning means which is forced into contact with a film which is for the thin film mirror being constructed, the tensioning means being positioned adjacent to and inwardly of at least one of the edges of the suction chamber such that tension in the film parallel to the edge is increased by the tensioning means moving the film in a direction into the suction chamber; and
(iii) providing holding means for
(a) holding the tensioning means;
(b) locally adjusting holding pressure on the tensioning means; and
(c) feeding the tensioning means into position after the film has been attached to the suction chamber.
The method of the present invention is able to provide a thin film mirror having an improved edge quality, without the need to substantially increase the size of the thin film mirror.
The method of constructing the thin film mirror may be one in which the tensioning means is a rod. Preferably the rod is of circular cross section but the rod may be of any suitable and appropriate cross sectional shape if desired, for example rectangular, square, elliptical, triangular, or combinations of shapes. Generally the tensioning means should present a smooth face to the film so as not to damage the film. Where the tensioning means is a rod of circular cross section, then the rod may be a tube or a solid rod. The tensioning means may be semi-rigid, for example a semi-rigid tube.
The holding means may have a first portion for extending over the tensioning means, and a second portion for extending under the suction chamber. The holding means may be of any suitable and appropriate shape. The holding mans may operate like a clamp.
The present invention also provides a thin film mirror when produced by the method of the invention.